The app Files was introduced to iPhone and iPad with iOS 11 2017, and it took very much needed file access to iOS. In typical Apple fashion, the real power is very hidden. To change files to the Finder for iOS.
If you've ever ventured into the Files app, you've no doubt seen it in its most basic form. Before adding any additional sources of files ̵1; which files called "sites" things can look pretty sparse. Certainly, if you browse files stored locally on your device, it is much better than what we previously had, but things become very interesting when you start adding more sites.
Places are storage areas that iOS apps can present in the Files app, allowing you to dive into the programs and data they have stored, all from within files. It doesn't sound all that great until you think about which apps support files. Dropbox, Google Drive and OneDrive will soon be remembered, and with one of those added as a site, you can browse the contents of the cloud storage providers directly from files. Everything is synchronized just as it should from apps, so if you add a file to your Dropbox folder on a Mac, it will appear in the Files app and vice versa.
This is the kind of integration that goes a long way to making Files work as a Finder, but there is also more.
Apps that you would not normally associate with storage can also present information to files. An example would be the excellent Keep It, an app for organizing files like PDFs, images and more. Keep It also presents everything it holds as a place in files, giving you instant access to everything without opening the app. Do you want to add a file to keep it processed at a later time? Just drop it into the Keep It site in files, and it's waiting for you whether you open the app on your iPhone, iPad or even your Mac.
So how do you do all this magic happen? It is surprisingly easy!
Add a location to files
To get started, open the Files app on your iPhone or iPad.
To start adding a new mode, press the "Edit" icon in the upper right corner of the screen.
You will now see a list of potential places with switches next to them. All apps that are installed and supported also File locations are shown here. To activate a location, turn its gear to the "On" position. You can also re-place places by dragging their grip handle up and down the list.
After activating the places you want, press "Done" in the upper right corner. 
That's all there is. To start browsing a location, tap it in Files. You will see the folder structure within the site, and you can work with all your files from there.